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Abstract:

An imaging lens includes a first lens having negative refractive power; a
second lens having positive refractive power; a third lens having
positive refractive power; a stop; and a fourth lens having positive
refractive power arranged in the order from an object side to an image
plane side. The first lens has an image plane-side surface having a
positive curvature radius. The second lens has an image plane-side
surface having negative curvature radius. The third lens has an image
plane-side surface having a negative curvature radius. The fourth lens
has an object-side surface having a positive curvature radius and an
image plane-side surface having a negative curvature radius. The first
lens has a specific focal length and a specific Abbe's number to satisfy
specific conditional expressions.

Claims:

1. An imaging lens comprising: a first lens having negative refractive
power; a second lens having positive refractive power; a third lens
having positive refractive power; a stop; and a fourth lens having
positive refractive power, arranged in this order from an object side to
an image plane side, wherein said first lens is formed in a shape so that
a surface thereof on the image plane side have a positive curvature
radius, said second lens is formed in a shape so that a surface thereof
on the image plane side have a negative curvature radius, said third lens
is formed in a shape so that a surface thereof on the image plane side
has a negative curvature radius, said fourth lens is formed in a shape so
that a surface thereof on the object side has a positive curvature radius
and a surface thereof on the image plane side has a negative curvature
radius, and said first lens has a focal length f1 and an Abbe's number
νd1 so that the following conditional expressions are satisfied:
-75<f1/f<-5.0 45<νd1<70 where f is a focal length of a
whole lens system.

2. The imaging lens according to claim 1, wherein said first lens is
formed in an aspheric shape so as the first lens has refractive power
increasing from an optical axis thereof toward a periphery thereof.

3. The imaging lens according to claim 1, wherein each of said first
lens, said second lens, and said third lens has a focal length three
times greater than that of the fourth lens.

4. The imaging lens according to claim 1, wherein said fourth lens has a
focal length f4 so that the following conditional expression is
satisfied: 1.0<f4/f<2.5 where f is the focal length of the whole
lens system.

5. The imaging lens according to claim 1, wherein said second lens and
said third lens have a composite focal length f23, and said fourth lens
has a focal length f4 so that the following conditional expression is
satisfied: 0.2<f4/f23<1.0.

6. The imaging lens according to claim 1, wherein said first lens, said
second lens, and said third lens have a composite focal length f123 so
that the following conditional expression is satisfied:
2.0<f123/f<5.0 where f is the focal length of the whole lens
system.

7. The imaging lens according to claim 1, wherein said first lens is
situated away from the second lens by a distance dA on an optical axis
thereof so that the following conditional expression is satisfied:
0.3<dA/f<1.0 where f is the focal length of the whole lens system.

8. The imaging lens according to claim 1, wherein said third lens has an
Abbe's number νd3 and said fourth lens has Abbe's number νd4 so
that the following conditional expressions are satisfied:
20<νd3<40 45<νd4<70

Description:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to an imaging lens for forming an
image of an object on an imaging element such as a CCD sensor and a CMOS
sensor. In particular, the present invention relates to an imaging lens
suitable for mounting in a relatively small camera such as a cellular
phone, a digital still camera, a portable information terminal, a
security camera, a vehicle onboard camera, a network camera, a video
conferencing camera, a fiberscope, and an encapsulated endoscope.

[0002] In these years, there have been available some vehicles equipped
with a plurality of cameras for a purpose of enhancing convenience and
security. For example, in case of a vehicle equipped with a backup camera
to take an image behind the vehicle, since a driver can see the rear view
of the vehicle on a monitor upon backing up the vehicle, the driver can
safely move the vehicle backward without hitting an object even if any,
although such an object is not visible from the driver due to shadow of
the vehicle. Such a camera mounted on a vehicle, i.e., a so-called
onboard camera, is expected to be continuously on demand.

[0003] The onboard cameras are often accommodated in a relatively small
space such as in a backdoor, a front grill, a side mirror, and inside of
the vehicle. For this reason, an imaging lens to be mounted in the
onboard camera is required to have a compact size. Further, the onboard
camera is required to be compatible with a high resolution resulting from
a high-pixel density imaging element, and to have a wide angle to be
compatible with a wide imaging range. However, it is difficult to attain
a small size and compatibility with the high resolution while
satisfactorily correcting aberrations, and further attain a wide imaging
angle. For example, when a size of an imaging lens is reduced, refractive
power of each lens tends to become stronger. Accordingly, it is difficult
to satisfactorily correct aberrations. Therefore, upon actually designing
the imaging lens, it is important to satisfy those demands in a balanced
manner.

[0004] As a wide-angle imaging lens that has a wide imaging angle, for
example, there is known an imaging lens described in Patent Reference.
The imaging lens includes a first lens that has a shape of a meniscus
lens directing a convex surface thereof to an object side and is
negative; a second lens that has a shape of a meniscus lens directing a
concave surface thereof to the object side and is positive; a third lens
that is positive; and a fourth lens that is positive, arranged in the
order from the object side.

[0006] According to the imaging lens disclosed in Patent Reference, the
second lens is made of a material having Abbe's number between 23 and 40,
and the third lens is made of a material having Abbe's number between 50
and 85. Furthermore, according to the imaging lens, a ratio (f/D) of a
focal length f of the whole lens system and a distance D from an incident
surface on the object side to an image-forming surface is restrained
within certain ranges. Accordingly, it is possible to obtain a wide angle
of view and a small size, and also satisfactorily correct a chromatic
aberration.

[0007] According to the imaging lens disclosed in Patent Reference,
although the number of lenses that compose the imaging lens is as few as
four, an imaging angle of view is wide and it is also possible to
relatively satisfactorily correct aberrations. However, demands for such
a wide-angle imaging lens have become diversified each year, and
especially in these years, there are strong demands for being capable of
manufacturing the imaging lens inexpensively, i.e., an imaging lens that
is easy to assemble with high productivity, as well as the demands to be
compatible to high-resolution imaging elements and to have a small size.

[0008] In case of the conventional wide-angle imaging lens including the
imaging lens disclosed in Patent Reference, the first lens has very
strong negative refractive power relative to other lenses in order to
attain a wide angle of view. For this reason, a curvature radius of an
image plane-side surface of the first lens is small, and thereby a
so-called semispherical ratio is close to 1.0 (semispherical shape),
which results in poor workability of the lens.

[0009] Further, the image plane-side surface of the first lens is
frequently coated with an antireflection coating or the like, and there
is a serious issue of insufficient coating a periphery of the lens
surface in case of a lens having the semispherical ratio near 1.0
described above. Furthermore, in case of the imaging lens, in which the
first lens has strong refractive power and has the semispherical ratio
near 1.0, the sensitivity to deterioration of image-forming performance
due to decentering (eccentricity), tilting, etc. occurred upon
manufacturing of the imaging lens, i.e., production error sensitivity, is
high, and there is a limit by itself for reduction of the manufacturing
cost.

[0010] Here, those issues are not unique to an imaging lens for mounting
on an onboard camera, but are common in imaging lenses for mounting in
relatively small cameras, such as cellular phones, digital still cameras,
portable information terminals, security cameras, network cameras, video
conferencing cameras, fiberscopes, and encapsulated endoscopes.

[0011] In view of the above-described problems of conventional techniques,
there is provided an invention, an object of which is to provide an
imaging lens that has a wide imaging angle of view despite of a small
size thereof and can suitably reduce the manufacturing cost.

[0012] Further objects and advantages of the present invention will be
apparent from the following description of the present invention.

SUMMARY OF THE INVENTION

[0013] In order to attain the objects described above, according to a
first aspect of the invention, an imaging lens includes a first lens that
has negative refractive power; a second lens having positive refractive
power; a third lens having positive refractive power; a stop; and a
fourth lens having positive refractive power, arranged in the order from
an object side to an image plane side. The first lens has an image
plane-side surface having a positive curvature radius. The second lens
has an image plane-side surface having negative curvature radius. The
third lens has an image plane-side surface having a negative curvature
radius. The fourth lens has an object-side surface having a positive
curvature radius and an image plane-side surface having a negative
curvature radius. Furthermore, when the whole lens system has a focal
length f, the first lens has a focal length f1, and the first lens has
Abbe's number νd1, the imaging lens of the invention having the
above-described configuration satisfies the following conditional
expressions (1) and (2):

-75<f1/f<-5.0 (1)

45<νd1<70 (2)

[0014] When the imaging lens satisfies the conditional expression (1), it
is possible to restrain a chromatic aberration, a distortion, an
astigmatism, and a field curvature within satisfactory ranges
respectively in a balanced manner, while attaining downsizing of the
imaging lens. When the value exceeds the upper limit of "-5.0", the
negative refractive power of the first lens is strong relative to the
whole lens system, which is advantageous for correction of the
distortion, the astigmatism, a chromatic aberration of magnification,
etc. However, since an axial chromatic aberration is insufficiently
corrected (a focal position at a short wavelength moves to the object
side relative to a focal position at a reference wavelength), it is
difficult to obtain satisfactory image-forming performance. Furthermore,
since incident pupil moves to the object side, a back focal length is
long, so that it is difficult to downsize the imaging lens.

[0015] On the other hand, when the value is below the lower limit of
"-75", the negative refractive power of the first lens is weak relative
to the whole lens system, the chromatic aberration of magnification is
insufficiently corrected, and negative distortion increases. Furthermore,
since periphery of the image-forming surface curves to the object side,
it is difficult to restrain the field curvature within a satisfactory
range. Therefore, also in this case, it is difficult to obtain
satisfactory image-forming performance.

[0016] In most cases, the first lens of a conventional wide-angle imaging
lens has strong refractive power relative to that of the whole lens
system. Also in case of the imaging lens of the invention, the first lens
has negative refractive power in order to attain a wide angle of view.
However, as shown in the conditional expression (1), the first lens has
weak refractive power relative to that of the whole lens system.
Therefore, the curvature radius of the image plane-side surface of the
first lens is large, and so-called hemispheric ratio is away from 1.0.
Therefore, an image plane-side concave surface of the first lens has a
half-elliptic shape that is recessed in a direction perpendicular to the
optical axis. For this reason, according to the imaging lens of the
invention, it is easy to evenly apply coating such as anti-reflection
coating, and it is possible to improve the yield upon manufacturing the
imaging lens. Furthermore, since the first lens has relatively weak
refractive power, it is possible to suitably reduce sensitivity
(production error sensitivity) to deterioration of the image-forming
performance due to decentering (eccentricity), tilting, etc. occurred
upon manufacturing the imaging lens.

[0017] When the imaging lens satisfies the conditional expression (2), it
is possible to effectively restrain occurrence of the chromatic
aberration. Having Abbe's number of the first lens greater than the lower
limit of "45", it is possible to effectively restrain the chromatic
aberration generated in the first lens. Furthermore, generally in case of
a wide-angle imaging lens, the first lens has the largest effective
diameter. Having the Abbe's number of the first lens smaller than the
upper limit of "75", it is not necessary to use an expensive material,
and it is possible to suitably attain reduction of manufacturing cost for
the imaging lens.

[0018] The imaging lens having the above-described configuration may be
preferably configured to further satisfy the following conditional
expression (1-A). When the imaging lens satisfies the conditional
expression (1-A), it is possible to satisfactorily correct aberrations
and more effectively restrain manufacturing cost by reducing
unsatisfactory application of coating such as anti-reflection coating and
reduction of the production error sensitivity.

-50<f1/f<-10 (1-A)

[0019] According to a second aspect of the present invention, in the
imaging lens having the above-described configuration, the first lens may
be preferably formed as an aspheric shape so as to have stronger negative
refractive power as it goes from the optical axis towards the periphery.
As described above, according to the first aspect of the present
invention, the first lens has weaker refractive power than conventional
one. For this reason, how to correct the filed curvature is a key. From
this point of view, in case of the imaging lens of the invention, since
the first lens has strong refractive power at the periphery than that
near the optical axis, correction at the periphery of the image-forming
surface is satisfactorily made, and aberrations including the field
curvature are satisfactorily corrected.

[0020] According to a third aspect of the present invention, in the
imaging lens having the above-described configuration, the focal length
of the first lens, the focal length of the second lens, and the focal
length of the third lens may be preferably longer than three times the
focal length of the fourth lens, respectively.

[0021] As well known, an imaging element of a CCD sensor, CMOS sensor, or
the like has a predetermined so-called "chief ray angle (CRA)", a range
of an incident angle of a light beam that can be taken by the sensor.
Restraining the incident angle of a light beam emitted from the imaging
lens to the image plane within the CRA range, it is possible to suitably
restrain generation of shading, which is a phenomenon of having dark
periphery in an image. For this reason, with the fourth lens, which is
disposed most closely to the image plane, has the strongest refractive
power, the imaging lens of the invention can have a configuration capable
of suitably restraining the incident angle of a light beam emitted from
the imaging lens to the imaging element.

[0022] As a result of optical simulations, it was found that it is
achievable to obtain a wide angle of view, corrections of aberrations,
etc. in a balanced manner while attaining downsizing of the imaging lens,
by having the focal length of the first lens, the focal length of the
second lens, and the focal length of the third lens longer than three
times the focal length of the fourth lens. Furthermore, since three out
of the four lenses have weak refractive powers as described above, it is
possible to even more effectively reduce the production error
sensitivity.

[0023] In view of reduction of the manufacturing cost, it is preferred to
form each lens from a resin material. However, for example, in case of an
imaging lens for an onboard camera to be mounted in an automobile, since
it is not rare that a temperature inside the vehicle exceeds 70°
C. in the midsummer hot sun, it is a critical issue upon designing to
restrain fluctuation of the focal length due to the temperature changes.
For such imaging lens used under severe environment, it is conventionally
necessary to form each lens from a glass material, which results in
increase of the manufacturing cost.

[0024] Therefore, for the imaging lens having the above-described
configuration, it is preferred to form the fourth lens from a glass-based
material. As described above, according to the imaging lens of the
invention, only the fourth lens has strong refractive power. For this
reason, forming the fourth lens, which has strong refractive power, from
a glass-based material, it is possible to minimize the fluctuation of the
focal length of the imaging lens due to temperature changes in the
surrounding environment. On the other hand, three lenses from the first
to the third lenses have relatively weak refractive power, so that the
fluctuation of the focal length due to temperature changes is small.
Accordingly, it is possible to not only form those three lenses from
glass-based materials, but also form from resin materials.

[0025] According to a fourth aspect of the present invention, when the
fourth lens has a focal length f4, the imaging lens having the
above-described configuration may be preferably configured to satisfy the
following conditional expression (3) :

1.0<f4/f<2.5 (3)

[0026] When the imaging lens satisfies the conditional expression (3), it
is possible to satisfactorily correct distortion while attaining
downsizing of the imaging lens. Furthermore, when the imaging lens
satisfies the conditional expression (3), it is also possible to suitably
restrain an incident angle of a light beam emitted from the imaging lens
to an imaging element within the CRA range. When the value exceeds the
upper limit of "2.5", since the fourth lens has weak refractive power,
although it is effective for correcting distortion, the axial chromatic
aberration is insufficiently corrected, so that it is difficult to obtain
satisfactory image-forming performance.

[0027] Furthermore, it is difficult to restrain an incident angle of a
light beam emitted from the imaging lens to the imaging element within
the CRA range. On the other hand, when the value is below "1.0", since
the fourth lens has relatively strong refractive power, it is easier to
restrain the incident angle of a light beam emitted from the imaging lens
to the imaging element within the CRA range. However, since the
distortion increases and the off-axis chromatic aberration of
magnification is insufficiently corrected, also in this case, it is
difficult to obtain satisfactory image-forming performance.

[0028] According to a fifth aspect of the present invention, when the
fourth lens has a focal length f4 and a composite focal length of the
second lens and the third lens is f23, the imaging lens having the
above-described configuration may be preferably configured to satisfy the
following conditional expression (4):

0.2<f4/f23<1.0 (4)

[0029] When the imaging lens satisfies the conditional expression (4), it
is possible to restrain an astigmatism, a chromatic aberration, and a
distortion within satisfactory ranges in a balanced manner. When the
value exceeds the upper limit of "1.0", among the lenses having positive
refractive power, the fourth lens has relative weak refractive power,
which is effective for correcting the off-axis chromatic aberration of
magnification, the axial chromatic aberration is insufficiently corrected
and astigmatic difference increases, so that it is difficult to obtain
satisfactory image-forming performance. On the other hand, when the value
is below "0.2", among the lenses having positive refractive power, the
fourth lens has relatively strong refractive power, and the off-axis
chromatic aberration of magnification is insufficiently corrected and a
sagittal image surface of the astigmatism curves to the object side.
Furthermore, since the distortion also increases, it is difficult to
obtain satisfactory image-forming performance.

[0030] According to a sixth aspect of the present invention, the when a
composite focal length from the first lens to the third lens is f123, the
imaging lens having the above-described configuration may be preferably
configured to satisfy the following conditional expression (5):

2.0<f123/f<5.0 (5)

[0031] When the imaging lens satisfies the conditional expression (5), it
is possible to satisfactorily correct the chromatic aberration, while
attaining downsizing of the imaging lens. Furthermore, when the imaging
lens satisfies the conditional expression (5), it is also possible to
suitably restrain the incident angle of a light beam emitted from the
imaging lens to the imaging element within the CRA range. When the value
exceeds the upper limit of "5.0", since the composite refractive power of
the first lens to the third lens, which are arranged on the object side
relative to the stop, the back focal length is long and it is difficult
to attain downsizing of the imaging lens.

[0032] Furthermore, the chromatic aberration of magnification is
insufficiently corrected near periphery of the image and it is difficult
to obtain satisfactory image-forming performance. On the other hand, when
the value is below "2.0", although it is effective for downsizing of the
imaging lens and satisfactory correction of the chromatic aberration of
magnification, it is difficult to restrain the incident angle of a light
beam emitted from the imaging lens to the imaging element within the CRA
range.

[0033] According to a seventh aspect of the present invention, when a
distance on an optical axis between the first lens and the second lens is
dA, the imaging lens having the above-described configuration may be
preferably configured to satisfy the following conditional expression
(6):

0.3<dA/f<1.0 (6)

[0034] When the imaging lens satisfies the conditional expression (6), it
is possible to satisfactorily correct the distortion and the astigmatism
while attaining downsizing of the imaging lens. When the value exceeds
the upper limit of "1.0", the size of the first lens is big, so that it
is difficult to attain downsizing of the imaging lens. Furthermore, an
off-axis chromatic aberration of magnification is insufficiently
corrected and it is difficult to correct periphery of a tangential image
surface, so that it is difficult to obtain satisfactory image-forming
performance. On the other hand, when the value is below the lower limit
of "0.3", although it is effective for downsizing of the imaging lens, a
sagittal image surface of the astigmatism curves to the object side, and
astigmatic difference increases. Furthermore, since the distortion
increases, it is difficult to obtain satisfactory image-forming
performance.

[0035] According to an eighth aspect of the present invention, when the
third lens has Abbe's number νd3 and the fourth lens has Abbe's number
νd4, the imaging lens may be preferably configured to satisfy the
conditional expressions (7) and (8):

20<νd3<40 (7)

45<νd4<70 (8)

[0036] When the imaging lens satisfies the conditional expressions (7) and
(8), it is possible to restrain the chromatic aberration within
satisfactory range. Forming the third lens and the fourth lens, which are
disposed across the stop from each other, from a material having Abbe's
number within the ranges indicated by the conditional expressions (7) and
(8), it is possible to satisfactorily correct the axial and off-axis
chromatic aberrations.

[0037] When a half angle of view of the lens system is ω, the
imaging lens having the above-described configuration preferably
satisfies "135°≦2ω". The imaging lens of the
invention is especially effective for an imaging lens that is required to
have an angle of view not smaller than 135°.

[0038] According to the imaging lens of the invention, it is possible to
provide a small-sized imaging lens that can suitably attain both a wide
angle of the imaging lens and reduction of manufacturing cost. Moreover,
according to the imaging lens of the invention, it is possible to provide
an imaging lens having less fluctuation in focal length due to
temperature changes of the surrounding environment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 1 according to an embodiment of
the invention;

[0040] FIG. 2 is an aberration diagram showing a lateral aberration of the
imaging lens of FIG. 1;

[0041] FIG. 3 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 1;

[0042] FIG. 4 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 2 according to the embodiment of
the invention;

[0043] FIG. 5 is an aberration diagram showing a lateral aberration of the
imaging lens of FIG. 4;

[0044] FIG. 6 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 4;

[0045] FIG. 7 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 3 according to the embodiment of
the invention;

[0046] FIG. 8 is an aberration diagram showing a lateral aberration of the
imaging lens of FIG. 7;

[0047] FIG. 9 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 7;

[0048] FIG. 10 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 4 according to the embodiment of
the invention;

[0049] FIG. 11 is an aberration diagram showing a lateral aberration of
the imaging lens of FIG. 10;

[0050] FIG. 12 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 10;

[0051] FIG. 13 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 5 according to the embodiment of
the invention;

[0052] FIG. 14 is an aberration diagram showing a lateral aberration of
the imaging lens of FIG. 13;

[0053] FIG. 15 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 13;

[0054] FIG. 16 shows a sectional view of a schematic configuration of an
imaging lens in Numerical Data Example 6 according to the embodiment of
the invention;

[0055] FIG. 17 is an aberration diagram showing a lateral aberration of
the imaging lens of FIG. 16; and

[0056] FIG. 18 is an aberration diagram showing a spherical aberration, an
astigmatism, and a distortion of the imaging lens of FIG. 16.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0057] Hereunder, referring to the accompanying drawings, an embodiment of
the present invention will be fully described.

[0058] FIGS. 1, 4, 7, 10, 13, and 16 are schematic sectional views of
imaging lenses in Numerical Data Examples 1 to 6 according to the
embodiment, respectively. Since a basic lens configuration is the same
among those Numerical Data Examples, the lens configuration of the
embodiment will be described with reference to the illustrative sectional
view of Numerical Data Example 1.

[0059] As shown in FIG. 1, the imaging lens of the embodiment includes a
first lens L1 having negative refractive power; a second lens L2 having
positive refractive power; a third lens L3 having positive refractive
power; an aperture stop ST; and a fourth lens L4 having positive
refractive power, arranged in the order from an object side to an image
plane side. Here, an infrared cutoff filter, a cover glass, or the like
may be provided between the fourth lens L4 and an image plane IM.

[0060] The first lens L1 is formed in a shape, such that a curvature
radius of an object-side surface thereof r1 and a curvature radius of an
image plane-side surface thereof r2 are both positive so as to have a
shape of a meniscus lens directing a convex surface thereof to the object
side near an optical axis X. In addition, the first lens L1 is formed as
an aspheric shape so as to have stronger refractive power as it is close
to the periphery from the optical axis X.

[0061] More specifically, the first lens L1 has stronger negative
refractive power as it goes to the periphery from a part that is around
70% of the maximum effective diameter. Here, the shape of the first lens
L1 is not limited to the shape of a meniscus lens directing a convex
surface thereof to the object side near the optical axis X. The first
lens L1 can have any shape as long as the curvature radius of the image
plane-side surface thereof r2 is positive, and can be shaped such that
the curvature radius r1 is negative, i.e. a shape of biconcave lens near
the optical axis X. Numerical Data Examples 1 and 2 are examples, in
which the shape of the first lens L1 is that of a meniscus lens directing
the convex surface thereof to the object side near the optical axis X,
and Numerical Data Examples 3 to 6 are examples, in which the shape of
the first lens L1 is that of a biconcave lens near the optical axis X.

[0062] The second lens L2 is formed in a shape such that a curvature
radius of an object-side surface thereof r3 and a curvature radius of an
image plane-side surface thereof r4 are both negative and is formed so as
to have a shape of a meniscus lens directing a concave surface thereof to
the object side near the optical axis X. Among them, the image plane-side
surface of the second lens L2 is formed as an aspheric surface directing
a concave surface thereof to the object side near the optical axis X and
directing a convex surface thereof to the object side at the lens
periphery. In short, the second lens L2 of the embodiment is formed as an
aspheric surface shape such that an image plane-side surface thereof has
an inflexion point and has a shape of a meniscus lens directing a concave
surface thereof to the object side near the optical axis X, and has a
shape of a biconcave lens at lens periphery that is away from the optical
axis X.

[0063] In case of the wide-angle imaging lens such as the one according to
the invention, it is important how to correct the curvature at the
periphery of the image plane for the wide angle of imaging coverage, in
order to obtain satisfactory aberration. In this view, with such aspheric
shape of the second lens L2, since the curvature at the periphery of the
image plane is suitably restrained, it is possible to satisfactorily
correct the field curvature. Here, the shape of the second lens L2 is not
limited to that of the meniscus lens directing a concave surface thereof
to the object side near the optical axis X. The second lens L2 can have
any shape, as long as the curvature radius of the image plane-side
surface thereof r4 is negative, and can be formed in a shape such that
the curvature radius r3 is positive, i.e. a shape of a biconvex lens near
the optical axis X.

[0064] The third lens L3 is formed in a shape such that a curvature radius
of an object-side surface r5 is positive and a curvature radius of an
image plane-side surface r6 is negative, so as to have a shape of a
biconvex lens near the optical axis X. The shape of the third lens L3 is
not limited to the shape of the biconvex lens near the optical axis X.
The third lens L3 can have any shape as long as the curvature radius of
the image plane-side surface r6 is negative and can be formed in a shape
such that the curvature radius r5 is negative, i.e. a shape of a meniscus
lens directing a concave surface to the object side near the optical axis
X. Numerical Data Examples 1 to 4 are examples for that the third lens L3
has a shape of a biconvex lens near the optical axis X, and Numerical
Data Examples 5 and 6 are examples for that the third lens L3 has a shape
of a meniscus lens directing a concave surface thereof to the object side
near the optical axis X.

[0065] According to the imaging lens of the embodiment, the focal length
of the first lens L1, the focal length of the second lens L2, and the
focal length of the third lens L3 are respectively longer than three
times the focal length of the fourth lens L4. In other words, when the
first lens L1 has a focal length f1, the second lens L2 has a focal
length f2, the third lens L3 has a focal length f3, and the fourth lens
L4 has a focal length f4, the imaging lens of the embodiment satisfies
the following conditional expressions.

f1>3×f4, f2>3×f4, and f3>3 ×f4

[0066] The fourth lens L4 is formed in a shape such that a curvature
radius of an object-side surface thereof r8 is positive and a curvature
radius of an image plane-side surface thereof r9 is negative, so as to
have a shape of a biconvex lens near the optical axis X.

[0067] Furthermore, the imaging lens of the embodiment satisfies the
following respective conditional expressions. Therefore, according to the
imaging lens of the embodiment, it is possible to suitably attain a wide
angle of the imaging lens and reduction of the manufacturing cost, as
well as satisfactorily correct an aberration in spite of the small size
thereof.

-75<f1/f<-5.0 (1)

45<νd1<70 (2)

1.0<f4/f<2.5 (3)

0.2<f4/f23<1.0 (4)

2.0<f123/f<5.0 (5)

0.3<dA/f<1.0 (6)

20<νd3<40 (7)

45<νd4<70 (8)

[0068] In the above conditional expressions:

[0069] f: Focal length of
whole lens system

[0070] f1: Focal length of a first lens L1

[0071] f23:
Composite focal length of a second lens L2 and a third lens L3

[0072] f4:
Focal length of a fourth lens L4

[0073] f123: Composite focal length from
the first lens L1 to the third lens L3

[0074] dA: Distance on an optical
axis between the first lens L1 and the second lens L2

[0075] νd1:
Abbe's number of the first lens L1

[0076] νd3: Abbe's number of the
third lens L3

[0077] νd4: Abbe's number of the fourth lens L4

[0078] The imaging lens of the embodiment preferably further satisfies the
following conditional expression (1-A):

-50<f1/f<-10 (1-A)

[0079] When the imaging lens satisfies the conditional expression (1-A),
the first lens L1 has weaker refractive power, so that the first lens L1
is formed in a shape having a small difference between the thickness on
an optical axis and the thickness of periphery, i.e. a shape having a
small change in the thickness from the lens center to the periphery. With
this configuration, it is possible to improve workability of the first
lens L1, and also it is easy to evenly apply antireflection coating from
the center part of the lens to the periphery. Here, Numerical Data
Example 1 and Numerical Data Examples 3 to 6 are examples that satisfy
the above conditional expression (1-A), and Numerical Data Example 2 is
an example that does not satisfy the conditional expression (1-A).

[0080] Here, it is not necessary to satisfy all of the respective
conditional expressions, and it is achievable to obtain an effect
corresponding to the respective conditional expression when any single
one of the conditional expressions is individually satisfied.

[0081] In the embodiment, any lens surface of the respective lenses is
formed as an aspheric surface. When the aspheric surfaces applied to the
lens surfaces have an axis Z in a direction of the optical axis X, a
height H in a direction perpendicular to the optical axis X, a conical
coefficient k, and aspheric coefficients A4, A6, A8,
A10, and A12, a shape of the aspheric surfaces of the lens
surfaces is expressed as follows:

[0082] Next, Numerical Data Examples of the imaging lens of the embodiment
will be described. In each Numerical Data Example, f represents a focal
length of the whole lens system, Fno represents an F number, and 2ω
represents an angle of view, respectively. In addition, i represents a
surface number counted from the object side, r represents a curvature
radius, d represents a distance between lens surfaces (surface spacing)
on the optical axis, nd represents a refractive index for a d line, and
νd represents Abbe's number for the d line, respectively. Here,
aspheric surfaces are indicated with surface numbers i affixed with *
(asterisk). In addition, total surface spacing from the object-side
surface of the first lens L1 to the image plane IM is indicated as L14.

[0085] FIG. 2 shows a lateral aberration that corresponds to a ratio H of
each image height to the maximum image height (hereinafter referred to as
"image height ratio H"), which is divided into a tangential direction and
a sagittal direction (which is the same in FIGS. 5, 8, 11, 14, and 17).
Furthermore, FIG. 3 shows a spherical aberration (mm), an astigmatism
(mm), and a distortion (%) of the imaging lens in Numerical Data Example
1, respectively. In the aberration diagrams, for the lateral aberration
diagrams and spherical aberration diagrams, aberrations at each
wavelength, i.e. a g line (435.84 nm), an e line (546.07 nm), and a C
line (656.27 nm) are indicated. In astigmatism diagram, an aberration on
a sagittal image surface S and an aberration on a tangential image
surface T are respectively indicated (which are the same in FIGS. 6, 9,
12, 15, and 18). As shown in FIGS. 2 and 3, according to the imaging lens
of Numerical Data Example 1, the aberrations are satisfactorily
corrected.

[0088] FIG. 5 shows the lateral aberration that corresponds to the image
height ratio H of the imaging lens in Numerical Data Example 2, and FIG.
6 shows a spherical aberration (mm), astigmatism (mm), and a distortion
(%), respectively. As shown in FIGS. 5 and 6, according to the imaging
lens of Numerical Data Example 2, the aberrations are also satisfactorily
corrected.

[0091] FIG. 8 shows the lateral aberration that corresponds to the image
height ratio H of the imaging lens in Numerical Data Example 3, and FIG.
9 shows a spherical aberration (mm), an astigmatism (mm), and a
distortion (%), respectively. As shown in FIGS. 8 and 9, according to the
imaging lens of Numerical Data Example 3, the aberrations are also
satisfactorily corrected.

[0094] FIG. 11 shows the lateral aberration that corresponds to the image
height ratio H of the imaging lens in Numerical Data Example 4, and FIG.
12 shows a spherical aberration (mm), astigmatism (mm), and a distortion
(%), respectively. As shown in FIGS. 11 and 12, according to the imaging
lens of Numerical Data Example 4, the aberrations are also satisfactorily
corrected.

[0097] FIG. 14 shows the lateral aberration that corresponds to the image
height ratio H of the imaging lens in Numerical Data Example 5, and FIG.
15 shows a spherical aberration (mm), an astigmatism (mm), and a
distortion (%), respectively. As shown in FIGS. 14 and 15, according to
the imaging lens of Numerical Data Example 5, the aberrations are also
satisfactorily corrected.

[0098] Next, the imaging lens of Numerical Data Example 6 will be
described. According to the imaging lens of Numerical Data Example 6, the
fourth lens L4 is made of a glass-based material. Therefore, according to
the imaging lens of Numerical Data Example 6, it is possible to suitably
restrain fluctuation of the focal length due to temperature changes in
the surrounding environment.

[0100] Accordingly, the imaging lens of Numerical Data Example 6 satisfies
the above-described conditional expressions. Therefore, according to the
imaging lens, it is possible to satisfactorily correct aberrations in
spite of the wide angle thereof.

[0101] FIG. 17 shows a lateral aberration that corresponds to the image
height ratio H of the imaging lens in Numerical Data Example 6, and FIG.
18 shows a spherical aberration (mm), an astigmatism (mm), and a
distortion (%), respectively. As shown in FIGS. 17 and 18, according to
the imaging lens of Numerical Data Example 6, the aberrations are also
satisfactorily corrected.

[0102] According to the imaging lens of the embodiment described above, it
is achievable to obtain an angle of view (2ω) that is not smaller
than 135°. For reference, the imaging lenses of Numerical Data
Example 1 to 6 attain the angles of view that are as wide as
135.0° to 168.0°.

[0103] Here, according to each Numerical Data Example, a surface of each
lens is formed as an aspheric surface, but if there is certain
flexibility in the total length of the imaging lens or required optical
performances, it is also possible to form all or a part of the lens
surfaces as spherical surfaces.

[0104] Accordingly, when the imaging lens of the embodiment is applied in
an optical system for mounting in cameras such as cellular phones,
digital still cameras, portable information terminals, security cameras,
onboard cameras, network cameras, video conferencing cameras,
fiberscopes, and capsulated endoscopes, it is achievable to obtain both
high functionality and downsizing of the cameras.

[0105] The invention may be applied in an imaging lens for mounting in a
device that is required to have a wide angle of view as an imaging lens
and satisfactory aberration correction performance, as well as to
minimize fluctuation of a focal length due to temperature changes in the
surrounding environment or other cause, e.g. a security camera or onboard
camera. Furthermore, the invention may be applied in an imaging lens for
mounting in a device, which is required to have a wide angle of view as
an imaging lens and to be inexpensive, for example a camera such as a
cellular phone, smartphone, network camera, and encapsulated endoscope.

[0106] The disclosure of Japanese Patent Application No. 2012-262095,
filed on Nov. 30, 2012, is incorporated in the application by reference.

[0107] While the present invention has been explained with reference to
the specific embodiments of the present invention, the explanation is
illustrative and the present invention is limited only by the appended
claims.